Commercial lightwave systems use optical fibers to carry large amounts of multiplexed digital data over long distances from a transmit terminal to a receive terminal. The maximum distance that the data can be transmitted in the fiber without amplification or regeneration is limited by the loss associated with the optical fiber (along with chromatic dispersion and optical nonlinearity). To transmit optical signals over long distances, lightwave systems may include a number of amplifiers located along the fiber route from the transmit terminal to the receive terminal. Each amplifier boosts the weak received signal to compensate for the transmission losses which occurred from the last amplifier. Optical amplifiers, such as rare earth doped optical fiber amplifiers, require a source of pump energy. In a rare earth doped optical fiber amplifier, for example, a dedicated pump laser is coupled to the doped fiber to excite the active medium (rare-earth element) within the amplifier. At the same time, a communication signal is passed through the doped fiber. The pumped fiber exhibits gain at the wavelength of the communication signal, providing the desired amplification.
The source of pump energy need not be co-located with the active medium because the transmission fiber is capable of carrying both the pump energy and the communication signal. In this case, the amplifier is said to be "remotely pumped."
A common active medium is an optical fiber doped with erbium ions. If the optical fiber is doped with erbium, for example, pump energy is provided at wavelengths of approximately 980 nm or 1485 nm, which coincides with absorption peaks of erbium. However, if energy is supplied by a pump laser operating at a wavelength of 1485 nm and remotely located from the doped fiber, spontaneously generated, Raman Stokes-shifted light is created in the transmission path and will deplete the pump energy available to pump the erbium-doped optical fiber. The diminished pump power limits the gain and output power of the remote amplifier and contributes to the noise of the amplifier. This limits the distance a signal can be transmitted and also limits the number of channels that may be employed in a wavelength division multiplexed transmission system.
It would therefore be desirable to reduce the pumping inefficiency caused by spontaneously generated Raman Stokes light.